A reaction between benzene and propylene gives cumene. The oxidation of cumene results in cumene hydroperoxide. The cumene hydroperoxide is acid decomposed into phenol and acetone. A combination of these known reactions is the cumene process which is currently a mainstream method for the production of phenol.
The cumene process gives acetone as a by-product. This by-production is advantageous when both phenol and acetone are demanded. However, if the amount of by-product acetone is in excess of demand, the economic efficiency can be deteriorated due to the price difference between acetone and propylene which is a starting material. Methods have been then proposed in which the by-product acetone is converted into propylene through various reactions and is reused as a material in the cumene process.
Acetone is readily hydrogenated into isopropyl alcohol. A process has been then proposed in which isopropyl alcohol thus obtained is intramolecularly dehydrated into propylene and the propylene is reacted with benzene to give cumene. That is, acetone is reused as a material in the cumene process by being converted into propylene through reactions in two stages (Patent Literature 1).
Further, Patent Literatures 2 and 3 propose methods for producing propylene from acetone and hydrogen in one stage, namely, through a single reaction step. In order to implement the reuse of acetone on an industrial level through such a one-stage reaction, it is necessary not only that the process be a practical process capable of producing propylene from acetone with high activity and high selectivity but also that the catalyst used in the process be easily available or readily producible at low cost. For example, phosphotungstates described in Patent Literature 3 as examples of heteropoly acid salts are allegedly effective for catalyzing the dehydration reaction of isopropyl alcohol. However, the production of such phosphotungstates entails multiple reaction steps. Further, the establishment of a practical method capable of converting acetone into propylene as well as of producing olefins from corresponding general ketones with high selectivity is valuable in various fields of industry other than the phenol industry.
For example, Patent Literature 4 describes a method in which propylene is obtained in one stage through the hydrogenation of acetone at 400° C. in the presence of a Cu (25%)-ZnO (35%)-Al2O3 (40%) catalyst. However, the acetone conversion is as low as 89% in spite of the fact that the reaction temperature is high at 400° C. Further, the propylene selectivity obtained by this method is as low as 89% because of the side reaction hydrogenating the produced propylene into propane. According to the findings by the present inventors, it has been confirmed that the hydrogenation of acetone into propylene in the presence of a mixed catalyst including a hydrogenation catalyst and a general dehydration catalyst can be accompanied by the Aldol condensation of acetone by the dehydration catalyst with the result that the formed Aldol reaction product can further undergo a dehydration reaction, a decomposition reaction and a hydrogenation reaction. That is, by-products are likely to be derived from acetone that is a starting material. Further, the use of a general dehydration catalyst can induce other reactions such as the oligomerization of formed propylene. Thus, the present inventors consider that the design and selection of a catalyst, in particular a dehydration catalyst, are the technical key to successfully producing propylene from acetone and hydrogen.